This invention relates generally to a gas mixer for mixing two gas streams. The disclosure is particularly suited to a gas mixer for mixing a hydrocarbon-containing gas stream with an oxygen-containing gas stream. The disclosure features a design of a gas mixer which minimizes the potential for ignition of the two gases in the mixer. An example of where this invention has utility is gas mixers used in the industrial production of ethylene oxide.
The chemical compound ethylene oxide (chemical formula C2H4O) is an important industrial chemical used as an intermediate in the production of ethylene glycol (the main component of automotive antifreeze) and other chemicals. Ethylene oxide is also used as a sterilant for foods and medical supplies. It is a colorless flammable gas at room temperature, and can be cooled and stored as a liquid.
Ethylene oxide first achieved industrial importance during World War I as a precursor to both ethylene glycol and the chemical weapon mustard gas. In 1931, Theodore Lefort, a French chemist, discovered a means to prepare ethylene oxide directly from ethylene and oxygen, using silver as a catalyst. Since 1940, almost all ethylene oxide produced industrially has been made using this method.
In current industrial processes, ethylene oxide is produced when ethylene (CH2═CH2) and oxygen (O2) react on a silver catalyst at 200-300° C. showing large Ag nanoparticles supported on Alumina. Typically, chemical modifiers such as chlorine are also included. Pressures used are in the region of 1-2 MPa. The chemical equation for this reaction is:CH2═CH2+½O2→C2H4O
In ethylene oxide production systems, a gas mixer is used to mix the hydrocarbon and oxygen gas streams just upstream of the reaction chamber where the silver catalyst is present. The gas mixer is typically constructed in the form of a vessel or pipe. The vessel includes an inlet manifold for each of the two gases. The vessel is sometimes constructed with a main outer pipe containing the hydrocarbon-containing gas stream and internal concentric tubes or “fingers” which contain the oxygen stream. Mixing occurs at the point where the internal tubes end, where the oxygen gas flowing out of the fingers meets the main stream of hydrocarbon-containing gas flowing in the outer tube. This basic design is described in U.S. Pat. No. 3,706,534.
The art has long recognized that there is a risk of ignition of a hydrocarbon-containing gas stream (e.g., a stream of gas containing for example ethylene mixed with other hydrocarbon gases) at the point where it is combined with an oxygen gas in a gas mixer. Ignition can occur when a particle (e.g. a piece of sand, rust or pipe scale) entrained in the hydrocarbon or oxygen gas stream strikes a metallic surface in the mixer, e.g., the wall of the mixer, thereby producing a spark. If the spark occurs in the hydrocarbon stream in the highly flammable zone e.g., at, or close to, the point of mixing of the two gas streams, ignition can occur. The ignition damages the gas mixer and also requires an interrupt of production to suppress the ignition and allow the gas mixer to cool before recommencing production. The flammable region is confined to the mixing zone of the two gases. The hydrocarbon-containing gas as well as the reactor feed blend are below the lower O2 flammability limit—i.e., too rich to burn.
The art has devised a variety of gas mixer designs. Some of the designs are specifically directed to reducing the risk of ignition of hydrocarbon and oxygen gas stream. The known prior art includes the following patent documents, in addition to the above-cited '534 patent: U.S. Pat. Nos. 4,573,803; 3,702,619; 4,256,604; 4,415,508; 6,657,079; U.S. 2003/0021182; U.S. Pat. Nos. 3,518,284; 4,390,346; 3,237,923; 3,081,818; 2,614,616 and 6,840,256.
Production of ethylene oxide is but one example of an industrial process where a hydrocarbon-containing gas stream is mixed with an oxygen-containing gas stream, and where steps to avoid unintentional ignition of the hydrocarbon-containing gas stream are needed. Many other examples exist, some of which are mentioned in the above-cited patent documents. The features of this disclosure are applicable broadly to mixing of two gases, and should not be considered limited to hydrocarbon-oxygen gas mixing or the production of ethylene oxide.